Hello
There are so many problems that can arise, perhaps someone has met this one.
I have a gravity system -old boiler Ideal Standard 80K BTU with micro bore, pumped, two manifolds, 11 rads of differing sizes. But one in the study 'stopped' getting warm some time ago. The sytem seemed well balanced before then. I have drained syetem, which always had fernox in, added new inhibitor. the cool rad drained fine, no sludge.
The water is hot at the inlet of rad - but not at outlet. It will just not go through the rad, I have even put a new rad on. Althought it is microbore- I replaced the in/out valves for normal valves each side of rad so that thermostatic valves could be fitted.
So all the house is fine , but one room refuses to heat up. It is probably the furthest away from the manifold. Could the return pipe be blocked?, would that be strange? no other rads are affected. I have turned down lockshileds on other rad dramatically. still no go
Frustrating- any thought much appreciated Thanks

No it wouldn't be strange - had a few jobs lately where the pipes have became blocked.

Firstly check pump speed setting, turning up if possible. Is the pump old or noisy. If yes then you could try replacing this first, as it may just be the pump isn't performing as it used to.

The easiest way to check the pipes for blockages, is to remove the radiator and open one valve at a time so that water can flow into a bucket, you may find that the water in the return pipe will not flow. If this happens undo the bottom nut from the radiator valve (carefully because if it just a faulty valve then you'll get wet and the water will not be clear) if the water still doesn't flow then there's a blockage in the pipes or at the manifold.

Thanks, I will try higher pump setting.
The problem is that the pipes are buried in the walls in the plaster, which would make quite a job re piping- unless I moved it elsewhere.
I will try the water test too.
Thanks

To any one reading this old thread.
Eventually, I took off the rad , after draining system, and blew compressed air up the return microbore pipe!
This resulted in a gurgling sound, followed by a fountain of blackish water some2 to 3 feet high emerging from the return pipe.
I had suitable buckets ready- but still hard to catch water jet.
I did this a couple of times, with more controlled effect.
Anyway, put the system together, added inhibitor, bled system and the rad works!
Hooray. But I find that the microbore is devilish hard to balance. Some rads temps drop by 20C, other 5C.
still all 12 rads get hot now, so that is something.

Microbore is always a problem, normal power flushing might not work, but there is powder flushing. In short they use an abrasive powder in the water when power flushing this scrapes off all the sludge. A very effective treatment on microbore.